This invention relates to an under-heater type furnace for maintaining a molten metal at a high temperature, and more particularly an under-heater type furnace for the purpose of maintaining a low melting point metal such as an aluminum alloy, for example, at a high temperature of 680.degree.-720.degree. C. after deoxidization treatment, slag removing treatment and etc. prior to diecasting.
In such a type of conventional furnaces, gas or oil burners or electric heaters are used as means for heating a molten metal such as an aluminum alloy. Also, conventional furnaces are of an upper-heater type and an under-heater type. Further, in such an under-heater type of furnaces, various modes of fixing the heating means are employed. For example, in a prior art furnace, both ends of a protective tube for housing therein a heater penetrates through and are fixed between opposite side walls of the furnace. In another conventional under-heater type furnace, only one end of a protective tube for a heater is fixed at a side wall of the furnace.
In the conventional under-heater type furnaces of the two-end fixing type, a protective tube for housing a heater is apt to be broken due to its heat expansion. Also, a seal between the protective tube and the furnace side wall is easily broken. Its construction is complicated, which results in difficult working and assembling.
In the conventional furnaces of one-end fixing type, it is very difficult to support a long protective tube. For such a reason, even if the protective tube is inclined, a sealing portion between the protective tube and the furnace side wall may break so as to cause a molten metal to be leaked. Accordingly, in practice, only a short protective tube is employed. Thus, heat efficiency is bad.